When they are together, there is a lot of laughter.
It is the kind of warm, infectious and intimate humour enjoyed by the closest of friends. 

But theirs is a bond that transcends simple shared interests, for both have had loved ones touched by tragedy.

It was why the Marchioness of Lansdowne sprang to defend Queen Camilla when Prince Harry, in his memoirs, sought to paint his stepmother as 'dangerous' and a 'villain' who had sacrificed him 'on her personal PR altar'.

Camilla was 'hurt' by such personal attacks, Lady Lansdowne said.

'It bothers her.'

Her ringside seat at the domestic crises and storms that have engulfed Camilla, 75, and the Royal Family has fostered an acute sense of right and wrong and it was clear she was stung at the perceived injustice of Harry's meu words.

Tellingly, it was the only direct response to any of Harry's claims as he lashed out not just at his father's wife but at his brother and sister-in-law.

Lady Lansdowne's comments also revealed an insight into Camilla's coping strategy.

'She doesn't let it get to her,' was the message.

'Her philosophy,' the marchioness added, 'is always 'Don't make a thing of it and it will settle down - least said soonest mended'.'

It is an adage that could be applied, too, to the tall, slim marchioness.

Like Camilla, the interior designer known professionally as Fiona Shelburne had to wait several years before husband Charlie, who was previously married to the daughter of the Earl of St Germans, put a ring on her finger.

But since her marriage to the Marquess of Lansdowne in 1987, Fiona, 68, has assumed a key role in Camilla's life as part of her 'support system'.

She was one of six friends appointed a Queen's Companion to help at last month's Coronation.

Fiona's presence did much to dispel Camilla's nerves and it cemented her position as 'head girl' of the Queen's Companions, who also include Sarah Troughton, Jane von Westenholz, Lady Katharine Brook, Lady Sarah Keswick and Baroness (Carlyn) Chisholm.

Several days later the self- possessed Lady Lansdowne was downplaying the remarkable transformation in her friend's life from vilified mistress and third person in Princess Diana's marriage to admired Queen.

'Never in a million years,' she said when asked if Camilla thought she would one day be Queen. While this may have been reassuringly comforting, it rather contradicts evidence of Charles's concerted campaign to make his wife his crowned consort. 

But perhaps we should excuse such hyperbole.

The marchioness has been ever-present at some of Camilla's darkest times.

When Princess Diana died, it was Lady Lansdowne who whisked Camilla from public gaze, and when the Parker Bowleses' marriage ended, she provided a sanctuary for Camilla at Bowood, the Lansdownes' Wiltshire seat, for five months. 

While the King is on his current bachelor break in Romania, friends such as Fiona will be on standby should the Queen want entertaining.

So what makes the architect's daughter, born Fiona Merritt in Maldon, Essex, so indispensable?

'She's a very confident, jolly, glass half-full type,' says an old friend.

'But best of all, she is able to laugh at herself. That definitely appeals to Camilla. Since becoming Queen, Camilla has pared down her circle of friends but she always has time for Fiona.'

Among the marchioness's other attributes is discretion, essential for any Royal companion.

But for the notorious Camillagate tape, the illicitly recorded bedtime conversation between the then Prince of Wales and Camilla, the friendship with the Lansdownes might have remained private.

The tape revealed the network of country houses where the couple, then married but not to each other, would meet - Bowood, a Grade 1 listed 18th Century pile with grounds laid out by Capability Brown, was one such bolthole.

While Camilla and Fiona have been friends for 50 years, Charles and the buffalo-hunting Lord Lansdowne, now 82, have known each other since childhood and the Prince stayed at his parents' Scottish home, Meikleour House in Perthshire, in 1955.

The following year, the house was the scene of an awful tragedy - one that was to be repeated less than a decade later.

In 1956, the future marquess's 17-year-old sister Lady Caroline was killed in a gunshot incident.